Patent Document 1: JP 04-27823 B
Patent Document 2: JP 03-27293 A
Patent Document 3: JP 3060227 B2
Patent Document 4: JP 05-176786 A
Patent Document 5: JP 07-107972 A
Patent Document 6: JP 10-101705 A
Patent Document 7: JP 09-309902 A
Patent Document 8: JP 10-298175 A
Patent Document 9: JP 2003-195 A
Non-Patent Document 1: Planta Med, 67 (1), 49-54, 2001
Flavonoids are aromatic compounds whose basic structure is a phenyl chroman skeleton (C6-C3-C6/A ring-C ring-B ring), and are classified into flavones, flavonols, flavanones, flavanonols, isoflavones, anthocyanins, flavanols, chalcones, aurones and the like in accordance with the difference in the C ring moieties. In recent years, as the elucidation of a variety of functionalities of natural components proceeds, the physiological functions of flavonoids have been noted of. However, many of flavonoids are slightly soluble in water, in food products, particularly in drinks and the like, the problems such as cloudiness, sedimentation and the like are caused during the production and storage, and this makes their industrial utilization difficult.
Hitherto, as known methods of solubilizing flavonoids, there are a method of converting isoflavone, rutin, hesperidin or the like into α-glycosyl isoflavonoid, α-glycosyl rutin, α-glycosyl hesperidin or the like by reacting α-glycosyl transferase with it (see Patent Documents 1-3), a method of obtaining catechin glycosides by reacting sucrose phosphorylase with a mixture of catechins and glycose-1-phosphoric acid or sucrose (Patent Document 4), a method of forming glycosides by solubilizing flavonoids in an alkali region of pH 8 or more and/or by addition of a cyclodextrin, and subjecting the solubilized product to glycotransfer with a cyclodextrin synthetic enzyme (Patent Document 5), a method of dissolving flavonoids in strong alkali solution and adding the resultant to a thickening polysaccharide solution (see Patent Document 6), a method of forming an inclusion product of isoflavone with cyclodextrin (Patent Documents 7 and 8) and the like are known.
However, any of the above-described methods is that for solubilizing a specific flavonoid, or that for enhancing the solubility of a specific flavonoid by structure conversion, and is not to provide a method for solubilizing flavonoids in general existing in nature as a basic technology. This is a problem from the viewpoint of versatility. In the methods of Patent Documents 7 and 8, since the solubility of cyclodextrin itself is not so high, the amount to be added is limited, and if a large amount is added, the cyclodextrin forms an inclusion compound with a flavoring agent, etc., and therefore the product design such as flavoring of foods, etc. are hindered.
On the other hand, isoflavones exist mainly in the form of glycosides in legumes and irises. Specifically, there are daidzin, genistin, glycitin, their malonyl glycosides such as 6″-O-malonyl daidzin, 6″-O-malonyl genistin and 6″-O-malonyl glycitin, their acetyl glycosides such as 6″-O-acetyl daidzin, 6″-O-acetyl genistin, 6″-O-acetyl glycitin, their aglycons such as daidzein, genistein and glycitein, and the like. While isoflavones are also in general slightly soluble in water, among them, the malonyl glycosides are known to have relatively soluble nature in water because they have dissociation groups on their side chains. However, it has not been known so far that the malonyl glycosides have an activity of solubilizing isoflavones, much less other flavonoids.
Saponins are classified mainly into steroid saponins and triterpenoid saponins from the viewpoint of their chemical structures. Quillaia saponin, soybean saponin, and enju saponin which are triterpenoid saponins are known to be natural emulsifiers for foods. Patent Document 9 discloses a technology for solubilizing a ginkgo leaf extract containing quercetin, etc. by using saponins together with glycerin, saccharides, etc., and teaches that quillaia saponin is preferred because quillaia saponin is particularly excellent in a surfactant capability. However, since there is a report that saponins are weak in an activity for solublizing a slightly soluble compound in water such as rutin, etc., and in general, should not be used as solubilizing agents (Non-Patent Document 1), it is unknown whether or not saponins have a high activity for solubilizing flavonoids and high versatility, even if their surfactant capability is high.